MOSFETs (Metal Oxide Semiconductor Field-Effect Transistors), can be classified into two types, namely, "analog" and "digital." The two types perform different functions. The digital type acts as a switch: it is either ON or OFF. The analog type acts as an amplifier: it amplifies a signal.
These two types are constructed differently: Analog transistors should have long channels with large surface area. Digital transistors should have channels as short as possible. However, since a short channel produces a large electric field along the channel's length (ie, between the source and the drain), the channel should be sufficiently long that the electric field does not exceed the breakdown value of the channel material. These differences will be explained in more detail.
Digital Transistors
The digital type is used as a switch by applying the proper voltage to the gate of the device, thereby causing a change in resistance of the channel material, thereby either (a) blocking current in the channel (with a high resistance), or (b) allowing it to flow (with a low resistance).
This alternation between high (OFF) and low (ON) resistance presents a design problem. On the one hand, the low-resistance state requires a short channel (ie, the distance between the drain and the source), because resistance=(length.times.resistivity)/cross-sectional area (1) as those terms are defined in FIG. 1. As equation (1) shows, the longer the channel, the higher the resistance.
On the other hand, the high-resistance state requires that the electric field extending between the source and drain remain below the critical value for the channel material.
For example, as in FIG. 2, assume (A) the voltage between the source and drain is five volts, and (B) the source-drain distance L is 1 .mu.M (one micro-meter). Consequently, the electric field is 5 volts per 10.sup.-6 meter, or 5.times.10.sup.6 volts per meter. This is a large electric field (in terms of volts per meter), even though the absolute voltage difference between the source and drain is small (ie, five volts).
The electric field must not be allowed to exceed the critical, breakdown voltage of the material.
Therefore, in a digital transistor, the channel is made as short as possible (for low ON resistance), subject to the requirement that the channel must be long enough to prevent the electric field in the OFF state from exceeding the critical value.
Analog Transistors
Analog MOSFETs require different channels: their channels should be long and wide. The channel must be long, in order to obtain high gain. [See, for example, S. M. Sze, Physics of Semiconductor Devices, (1969, John Wiley, ISBN 471 84290 7) chapter 7, section 3, p. 340 et seq].
The channel must be wide, in order to reduce a particular type of electrical noise, namely, "l/f noise." This type of noise is so-called because it has been experimentally found to be nearly inversely proportional to frequency, as the designation "l/f" indicates. Because of the inverse proportionality, much of the noise power is concentrated at low frequencies. If gain at low frequencies is important, then the "l/f noise" presents a problem.
It is theorized that this noise is caused by surface traps in the semiconductor material. The traps randomly absorb energy and then re-emit the energy as noise. One solution to the noise problem is to increase the surface area of the channel, which increases the population of surface traps. This solution reduces the "l/f noise" because, by increasing the number of traps, the number of traps available for absorption is increased, as well as the number available for emission. Restated, in a large population of traps, there is a greater likelihood of finding an available trap to absorb radiated noise.
Consequently, in an analog transistor, to reduce noise, the surface area of the channel is made large. To increase gain, the length of the channel is made long.
Conflict Between Analog and Digital Devices
Therefore, an inherent conflict between digital and analog transistors becomes apparent: Analog transistors should have long channels with large surface area. Digital transistors should have channels as short as possible (subject to the limit at which breakdown occurs.)
Exemplary Transistor Dimensions
Channel dimensions L (length) and W (width) are defined in FIG. 1. In a common digital transistor, the channel length lies between 0.7 and 1.5 microns (a micron equals 10.sup.-6 meter), and the channel width lies between 10 and 50 microns. In contrast, for an analog transistor, the channel length commonly lies between 16 and 25 microns, and the channel width commonly lies between 40 to 400 microns. Thus, based on these examples, in an analog transistor, the channel length is about 10 to 35 times longer, and the channel width ranges from equal to about 40 times wider, than the comparable dimensions in a digital transistor.